Refrigeration cycle apparatus

ABSTRACT

An object is to provide a refrigeration cycle apparatus capable of reducing freezing in a lower part of a heat exchanger in which drainage water tends to accumulate and reducing an amount of refrigerant in a refrigerant circuit. The refrigeration cycle apparatus includes the refrigerant circuit  1  connecting, by refrigerant pipes, a compressor, a first expansion device, and a first heat exchanger configured to serve as evaporator during heating operation. The first heat exchanger is provided with a first heat exchange unit and a second heat exchange unit connected to the first heat exchange unit in series in the refrigerant circuit. The first expansion device is connected in parallel with the second heat exchange unit in the refrigerant circuit, and the second heat exchange unit is placed at a position lower than a position of the first heat exchange unit.

TECHNICAL FIELD

The present disclosure relates to a refrigeration cycle apparatus and, in particular, to a connection between a heat exchanger configured to serve as evaporator and an expansion device.

BACKGROUND ART

During heating operation of an air-conditioning apparatus, which is a type of refrigeration cycle apparatus, high-temperature and high-pressure gas refrigerant discharged from a compressor is cooled by exchanging heat with indoor air through an indoor heat exchanger configured to serve as condenser, and undergoes a phase change to low-temperature and high-pressure liquid refrigerant. After that, the low-temperature and high-pressure liquid refrigerant is subjected to a phase change to low-temperature and low-pressure two-phase refrigerant by an expansion device. The two-phase refrigerant is heated by exchanging heat with air through an outdoor heat exchanger configured to serve as evaporator and undergoes a phase change to low-temperature and low-pressure gas refrigerant that is suctioned into the compressor. Then, the low-temperature and low-pressure gas refrigerant is compressed by the compressor and discharged again as high-temperature and high-pressure gas refrigerant.

When the temperature of outside air at which the outdoor heat exchanger is installed comes close to below freezing during heating operation of the air-conditioning apparatus, the surface temperature of the outdoor heat exchanger goes down to further below freezing for the maintenance of heat exchanging performance. At this time, frost may form on the outdoor heat exchanger. When increased frost forms on the outdoor heat exchanger, defrosting is needed. For example, the outdoor heat exchanger is defrosted by performing defrosting operation by a method such as causing hot gas to flow into the outdoor heat exchanger. Drainage water produced by defrosting usually falls in drops onto a drain pan for drainage. However, the stagnation of drainage of the drain pan or the effect of surface tension may cause water to accumulate in a lower end portion of the heat exchanger. In a state in which water accumulates in the heat exchanger, accumulated drainage water may freeze during heating operation to damage the outdoor heat exchanger. To address this problem, a known method prevents freezing in the outdoor heat exchanger with a heater installed to the drain pan.

For example, an air-conditioning apparatus disclosed in Patent Literature 1 reduces frost formation and freezing in a drain pan and a lower part of a heat exchanger by causing higher-pressure (temperature) refrigerant than refrigerant that flows through an upper portion of the heat exchanger to flow through a lower portion of the heat exchanger.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Utility Model Publication No. 5-44653

SUMMARY OF INVENTION Technical Problem

However, in the air-conditioning apparatus disclosed in Patent Literature 1, the cross-sectional area of a heat transfer pipe included in the heat exchanger may be reduced, for example, for further improvement in heat transfer performance of the heat exchanger or for a reduction in the amount of refrigerant that flows through the heat transfer pipe. Possible examples include reducing the outside diameter of a heat transfer pipe as which a circular pipe is used or forming many small-diameter-hole flow passages in a pipe that is flat in cross-section. In the air-conditioning apparatus according to Patent Literature, a problem exists in that a reduction in the cross-sectional area of a flow passage of a heat transfer pipe included in the heat exchanger undesirably leads to an increase in flow passage resistance in a lower portion of the heat exchanger in which the number of refrigerant flow passage bifurcations is small.

The present disclosure is intended to solve such a problem, and has an object to provide a refrigeration cycle apparatus capable of, even in the case of a small-diameter heat transfer pipe, reducing freezing in a lower part of a heat exchanger in which drainage water tends to accumulate.

Solution to Problem

A refrigeration cycle apparatus according to an embodiment of the present disclosure includes a refrigerant circuit connecting, by refrigerant pipes, a compressor, a first expansion device, and a first heat exchanger configured to serve as evaporator during heating operation. The first heat exchanger is provided with a first heat exchange unit and a second heat exchange unit connected to the first heat exchange unit in series in the refrigerant circuit. The first expansion device is connected in parallel with the second heat exchange unit in the refrigerant circuit, and the second heat exchange unit is placed at a position lower than a position of the first heat exchange unit.

Advantageous Effects of Invention

With such a configuration, an embodiment of the present disclosure makes it possible to reduce freezing in a drain pan and a lower part of a heat exchanger while, by reducing the cross-sectional area of a refrigerant flow passage of a heat transfer pipe of the heat exchanger, reducing the amount of refrigerant that flows through the refrigerant circuit.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a refrigerant circuit 1 of a refrigeration cycle apparatus 100 according to Embodiment 1.

FIG. 2 is a perspective view of a first heat exchanger 10 of the refrigeration cycle apparatus 100 according to Embodiment 1.

FIG. 3 is an explanatory diagram of the cross-sectional structure of the first heat exchanger 10 of FIG. 2.

FIG. 4 is an explanatory diagram of the structure of the first heat exchanger 10 according to Embodiment 1 as seen from the front.

FIG. 5 illustrates a cross-sectional view of a flat pipe as an example of a heat transfer pipe 20 for use in the first heat exchanger 10 of Embodiment 1.

FIG. 6 is a circuit diagram of a refrigerant circuit 101 of a refrigeration cycle apparatus 1100 as a comparative example of the refrigeration cycle apparatus 100 of Embodiment 1.

FIG. 7 is a perspective view of a first heat exchanger 110 of the refrigeration cycle apparatus 1100 according to the comparative example.

FIG. 3 is a diagram showing the characteristics of the refrigeration cycle apparatus 1100 of the comparative example during heating operation.

FIG. 9 is a diagram showing the characteristics of the refrigeration cycle apparatus 100 according to Embodiment 1 during heating operation.

FIG. 10 is an enlarged view of an A part of FIG. 9.

FIG. 11 is a circuit diagram of a refrigerant circuit 201 of a refrigeration cycle apparatus 200 according to Embodiment 2.

FIG. 12 is a perspective view of a first heat exchanger 210 of the refrigeration cycle apparatus 200 according to Embodiment 2,

FIG. 13 is a diagram showing the characteristics of the refrigeration cycle apparatus 200 according to Embodiment 2 during heating operation.

FIG. 14 is a diagram showing the characteristics of the refrigeration cycle apparatus 200 according to Embodiment 2 during heating operation.

FIG. 15 is a circuit diagram of a refrigerant circuit 301 of a refrigeration cycle apparatus 300 according to Embodiment 3.

FIG. 16 is a perspective view of a first heat exchanger 310 of the refrigeration cycle apparatus 300 according to Embodiment 3,

FIG. 17 is a diagram showing the characteristics of the refrigeration cycle apparatus 300 according to Embodiment 3 during heating operation.

FIG. 18 is a diagram showing the characteristics of the refrigeration cycle apparatus 300 according to Embodiment 3 during heating operation.

FIG. 19 is a circuit diagram of a refrigerant circuit 401 of a refrigeration cycle apparatus 400 according to Embodiment 4.

FIG. 20 is a perspective view of a first heat exchanger 410 of the refrigeration cycle apparatus 400 according to Embodiment 4.

FIG. 21 is a diagram showing the characteristics of the refrigeration cycle apparatus 400 according to Embodiment 4 during heating operation.

DESCRIPTION OF EMBODIMENTS

The following describes embodiments of refrigeration cycle apparatuses. Embodiments of the drawings are illustrative only, and are not intended to limit the present disclosure. Further, elements given identical reference signs in each drawing are identical or equivalent elements, and these reference signs are adhered to throughout the full text of the specification. Furthermore, a relationship in size between one element and another in the following drawings may be different from an actual one.

Embodiment 1

FIG. 1 is a circuit diagram of a refrigerant circuit 1 of a refrigeration cycle apparatus 100 according to Embodiment 1. The refrigeration cycle apparatus 100 shown in FIG. 1 is for example an air-conditioning apparatus. As shown in FIG. 1, the refrigeration cycle apparatus 100 includes a refrigerant circuit 1 by connecting a compressor 2, a four-way valve 7, a first heat exchanger 10, a first expansion device 5, and a second heat exchanger 3 by refrigerant pipes. For example, in a case in which the refrigeration cycle apparatus 100 is an air-conditioning apparatus, refrigerant flows through the refrigerant pipes, and switching between heating operation and cooling operation or defrosting operation is achieved by switching the flows of refrigerant with the four-way valve 7. Although Embodiment 1 illustrates an air-conditioning apparatus as the refrigeration cycle apparatus 100, the refrigeration cycle apparatus 100 is used for refrigeration applications or air-conditioning applications such as refrigerators, freezers, self-vending machines, air-conditioning apparatuses, refrigeration apparatuses, and water heaters.

The compressor 2, the second heat exchanger 3, the first expansion device 5, the first heat exchanger 10, and the four-way valve 7 form the refrigerant circuit 1, through which the refrigerant is allowed to circulate. The refrigeration cycle apparatus 100 performs a refrigerant cycle in which the refrigerant circulates throughout the refrigerant circuit 1 while undergoing phase changes. The compressor 2 compresses the refrigerant. The compressor 2 is for example a rotary compressor, a scroll compressor, a screw compressor, or a reciprocating compressor.

The first heat exchanger 10 is configured to serve as evaporator during heating operation of the refrigeration cycle apparatus 100, and is configured to serve as condenser during cooling operation of the refrigeration cycle apparatus 100. The first heat exchanger 10 is formed by a first heat exchange unit 11 and a second heat exchange unit 12. The second heat exchange unit 12 is placed at a position lower than a position of the first heat exchange unit 11.

The second heat exchanger 3 is configured to serve as condenser during heating operation of the refrigeration cycle apparatus 100, and is configured to serve as evaporator during cooling operation of the refrigeration cycle apparatus 100. Note, however, that the second heat exchanger 3 may be partially used as evaporator because of a drop in refrigerant temperature caused by a pressure loss in a pipe during heating operation. The first heat exchanger 10 and the second heat exchanger 3 are for example fin-and-tube heat exchangers, microchannel heat exchangers, finless heat exchangers, shell-and-tube heat exchangers, heat-pipe heat exchangers, double-pipe heat exchangers, or plate heat exchangers.

The first expansion device 5 expands and decompresses the refrigerant. The first expansion device 5 is for example an electric expansion valve capable of adjusting the flow rate of refrigerant. Instead of being an electric expansion valve, the first expansion device 5 may be a mechanical expansion valve in which a diaphragm is employed as pressure sensing portion, a capillary tube, or other devices.

The four-way valve 7 is configured to switch the flow passages of the refrigerant in the refrigeration cycle apparatus 100 and changes the direction of circulation of the refrigerant through the refrigerant circuit 1. The four-way valve 7 is switched during heating operation to connect a discharge port of the compressor 2 and the second heat exchanger 3 and connect a suction port of the compressor 2 and the first heat exchanger 10. Further, the four-way valve 7 is switched during cooling operation and dehumidifying operation to connect the discharge port of the compressor 2 and the first heat exchanger 10 and connect the suction port of the compressor 2 and the second heat exchanger 3.

An air-sending device 6 is disposed beside the first heat exchanger 10. Further, an air-sending device 4 is disposed beside the second heat exchanger 3. The first heat exchanger 10 is an outdoor heat exchanger mounted in an outdoor unit and, with the air-sending device 6 sending outside air into the first heat exchanger 10, allows heat exchange between the outside air and the refrigerant. Further, the second heat exchanger 3 is an indoor heat exchanger mounted in an indoor unit and, with the air-sending device 4 introducing indoor air into a housing of the indoor unit and sending the indoor air into the indoor heat exchanger, adjusts the temperature of the indoor air by allowing heat exchange between the indoor air and the refrigerant.

The configuration of the refrigerant circuit 1 of the refrigeration cycle apparatus 100 according to Embodiment 1 is described with reference to the flow of refrigerant in cooling and heating operational states. During cooling operation, the refrigerant discharged from the compressor 2 flows into the first heat exchange unit 11 of the first heat exchanger 10 through the four-way valve 7. The refrigerant having flowed out from the first heat exchange unit 11 bifurcates into two refrigerant flow passages one of which passes through the first expansion device 5 and the other of which passes through the second heat exchange unit 12. After that, the refrigerant having passed through the first expansion device 5 and the refrigerant having passed through the second heat exchange unit 12 merge into a flow of refrigerant that passes through the second heat exchanger 3 and the four-way valve 7 in sequence and that is suctioned into the compressor 2.

On the other hand, the refrigerant discharged from the compressor 2 flows into the second heat exchanger 3 through the four-way valve 7. The refrigerant having flowed out from the second heat exchanger 3 bifurcates into two refrigerant flow passages one of which passes through the first expansion device 5 and the other of which passes through the second heat exchange unit 12 of the first heat exchanger 10. After that, the refrigerant having passed through the first expansion device 5 and the refrigerant having passed through the second heat exchange unit 12 merge into a flow of refrigerant that passes through the first heat exchange unit 11 and the four-way valve 7 in sequence and that is suctioned into the compressor 2.

The refrigerant circuit 1 of the refrigeration cycle apparatus 100 includes a bifurcation 90 at which one of the refrigerant pipes bifurcates from the second heat exchanger 3 into one to the first heat exchanger 10 and the other one to the first expansion device 5. That is, no other expansion device is provided between the second heat exchanger 3 and the bifurcation 90.

(Structure of First Heat Exchanger 10)

FIG. 2 is a perspective view of the first heat exchanger 10 of the refrigeration cycle apparatus 100 according to Embodiment 1. FIG. 2 partially schematically shows refrigerant pipes connected to the first heat exchanger 10. As shown in FIG. 2, the first heat exchanger 10 includes the first heat exchange unit 11 and the second heat exchange unit 12. The second heat exchange unit 12 is placed at a position lower than a position of the first heat exchange unit 11.

The first heat exchange unit 11 and the second heat exchange unit 12 each include two heat exchange units arranged in series in the direction of flow of air flowing into the first heat exchanger 10. The first heat exchange unit 11 includes a first windward heat exchange unit 11 a as heat exchange unit located windward, and includes a first leeward heat exchange unit 11 b as heat exchange unit located leeward. The first windward heat exchange unit 11 a and the first leeward heat exchange unit 11 b are connected by a header 14 at end portions of the first windward heat exchange unit 11 a and the first leeward heat exchange unit 11 b. When the first heat exchanger 10 serves as evaporator, the refrigerant having flowed out from the first leeward heat exchange unit 11 b flows into the first windward heat exchange unit 11 a.

The second heat exchange unit 12 includes a second windward heat exchange unit 12 a as heat exchange unit located windward, and includes a second leeward heat exchange unit 12 b as heat exchange unit located leeward. The second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b are connected by the header 14 at end portions of the second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b. When the first heat exchanger 10 serves as evaporator, the refrigerant having flowed out from the second windward heat exchange unit 12 a flows into the second leeward heat exchange unit 12 b.

The first heat exchange unit 11 and the second heat exchange unit 12, which form the first heat exchanger 10, each include heat transfer pipes 20. The heat transfer pipes 20 are arranged in parallel with each other in a z direction shown in FIG. 2. In Embodiment 1, a z axis extends along the direction of gravitational force. Note, however, that the first heat exchanger 10 is not limited to one that is installed with the z direction aligned with the direction of gravitational force and, for example, may be installed with the z direction at a slant. That is, the plurality of heat transfer pipes 20 need only be arranged in parallel with each other in a vertical direction.

The header 14 includes an upper header 14 a connecting the first windward heat exchange unit 11 a and the first leeward heat exchange unit 11 b and a lower header 14 b connecting the second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b. The header 14, whose upper and lower headers 14 a and 14 b are integrally formed, has its interior partitioned into a plurality of spaces at least such that refrigerant of the first heat exchange unit 11 and refrigerant of the second heat exchange unit 12 do not mix.

The first windward heat exchange unit 11 a and the first leeward heat exchange unit 11 b do not need to be configured to be connected by the header 14. For example, a heat transfer pipe 20 that the first windward heat exchange unit 11 a has and a heat transfer pipe 20 that the first leeward heat exchange unit 11 b has may have their end portions connected by a U-shaped pipe. Similarly, the second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b do not need to be configured to be connected by the header 14, and a heat transfer pipe 20 that the second windward heat exchange unit 12 a has and a heat transfer pipe 20 that the second leeward heat exchange unit 12 b has may have their end portions connected by a U-shaped pipe.

In FIG. 2, the first heat exchange unit 11 includes a plurality of heat transfer pipes 20. The first windward heat exchange unit 11 a and the first leeward heat exchange unit 11 b each include a plurality of heat transfer pipes 20 in equal numbers, and are connected by the header 14. The plurality of heat transfer pipes 20 are arranged in parallel with each other in the z direction. Further, the plurality of heat transfer pipes 20 of the first windward heat exchange unit 11 a are connected to a windward collecting pipe 13 a at end portions of the plurality of heat transfer pipes 20 in a y direction. The plurality of heat transfer pipes 20 of the first leeward heat exchange unit 11 b are also connected to a leeward collecting pipe 13 b at end portions of the plurality of heat transfer pipes 20 in the y direction. The collecting pipes 13 a and 13 b are connected to refrigerant pipes included in the refrigerant circuit 1, and serve as inflow part or outflow part through which the refrigerant flows into or out from the first heat exchange unit 11. The collecting pipes 13 a and 13 b may be divided into a plurality of separate parts. For example, the upper three, middle three, and lower three of the plurality of heat transfer pipes 20 of the first leeward heat exchange unit 11 b may be connected to separate collecting pipes.

In FIG. 2, the second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b, which form the second heat exchange unit 12, each has one heat transfer pipe 20. Note, however, that the second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b may have a plurality of heat transfer pipes 20.

In Embodiment 1, the first windward heat exchange unit 11 a and the first leeward heat exchange unit 11 b of the first heat exchange unit 11 each have nine heat transfer pipes 20 arranged in the z direction, and the second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b of the second heat exchange unit 12 each have one heat transfer pipe 20 in the z direction. That is, the number of heat transfer pipes 20, arranged in parallel with each other, that the first windward heat exchange unit 11 a and the first leeward heat exchange unit 11 b of the first heat exchange unit 11 have is larger than the number of heat transfer pipes 20, arranged in parallel with each other, that the second windward heat exchange unit 12 a and the second leeward heat exchange unit 12 b of the second heat exchange unit 12 have. The numbers of heat transfer pipes 20 are not limited to these numbers. The numbers of refrigerant flow passages of the first heat exchange unit 11 and the second heat exchange unit 12 may be each set as appropriate. Note, however, that the number of refrigerant flow passages of the first heat exchange unit 11, which is located in an upper part, is larger than the number of refrigerant flow passages of the second heat exchange unit 12.

Actions of the first heat exchanger 10 during heating operation of the refrigeration cycle apparatus 100 are described here. High-pressure liquid refrigerant condensed through the second heat exchanger 3, which serves at least partially as condenser in the refrigeration cycle apparatus 100, bifurcates at the bifurcation 90 of the refrigerant pipes into two parallel flows of refrigerant that flow separately through a circuit connected to the first expansion device 5 and a bypass 95 connected to the second windward heat exchange unit 12 a. The refrigerant having flowed into the first expansion device 5 expands, that is, becomes decompressed and turns into low-temperature two-phase gas-liquid refrigerant. The refrigerant having flowed out from the first expansion device 5 merges with refrigerant having passed through the second leeward heat exchange unit 12 b. During passage of refrigerant through a device such as the first expansion device 5, a predetermined flow resistance is likely to be generated depending on the flow passage shape of the first expansion device 5, the amount of refrigerant that circulates through the refrigerant circuit 1, and the flow pattern of the refrigerant. The flow pattern of the refrigerant is a physical property of the refrigerant, and the refrigerant varies from state to state such as gas-phase flow, liquid-phase flow, and two-phase gas-liquid flow. Further, the flow resistance of the first expansion device 5 causes a pressure loss in the flow of refrigerant passing through the first expansion device 5. That is, the refrigerant having passed through the first expansion device 5 has reduced pressure.

Meanwhile, the refrigerant having flowed into the second windward heat exchange unit 12 a flows through the heat transfer pipe 20 and flows into the header 14 to move from the second windward heat exchange unit 12 a to the second leeward heat exchange unit 12 b. The header 14 has its interior space divided in correspondence with positions of the plurality of heat transfer pipes 20 arranged in parallel with each other in the z direction. The interior space of the header 14 is divided such that the lower header 14 b is formed in a lower part of the header 14. The lower header 14 b connects the heat transfer pipe 20 of the second windward heat exchange unit 12 a and the heat transfer pipe 20 of the second leeward heat exchange unit 12 b. The refrigerant having passed through the lower header 14 b flows into the second leeward heat exchange unit 12 b and, after having flowed through the heat transfer pipe 20, merges with the refrigerant having passed through the first expansion device 5.

As is the case with the aforementioned first expansion device 5, a heat transfer pipe 20 has a predetermined flow resistance while refrigerant is flowing through the heat transfer pipe 20. The flow resistance is generated depending on the shape of a flow passage in the heat transfer pipe 20, the amount of refrigerant that circulates through the refrigerant circuit 1, and the flow pattern of the refrigerant, and causes a pressure loss in the flow of refrigerant. The refrigerant having passed through the second heat exchange unit 12 and the refrigerant having passed through the first expansion device 5 merge and flow into the first heat exchange unit 11. The first heat exchange unit 11 has a plurality of heat transfer pipes 20. For example, the refrigerant is distributed at the lower collecting pipe 13 b to the plurality of heat transfer pipes 20 as parallel flows of refrigerant that flow separately into each of the heat transfer pipes 20. The parallel flows of refrigerant having flowed into the plurality of heat transfer pipes 20 pass through the first leeward heat exchange unit 11 b and flow into the first windward heat exchange unit 11 a through the upper header 14 a. The flows of refrigerant having passed through the plurality of heat transfer pipes 20 of the first windward heat exchange unit 11 a merge at the windward collecting pipe 13 a. That is, the separate flows of refrigerant through the plurality of refrigerant flow passages in the first heat exchange unit 11 merge at the windward collecting pipe 13 a and flow out from the first heat exchanger 10. The refrigerant having flowed out from the first heat exchanger 10 is suctioned into the compressor 2 through the four-way valve 7.

The ratio between the circulatory volumes of separate flows of refrigerant through the first expansion device 5 and the second heat exchange unit 12 is such a ratio that a pressure loss caused in the first expansion device 5 and a pressure loss caused in the second heat exchange unit 12 become equal. That is, the ratio between the circulatory volumes of refrigerant varies depending on the respective flow passage shapes of the first expansion device 5 and the second heat exchange unit 12 and a change in flow pattern entailed by decompression and heat balance of refrigerant. As one example, in a case in which the flow pattern of refrigerant is a single-phase liquid or gas state, the pressure loss ΔP is expressed by the following formula:

[Math.  1]                                        $\begin{matrix} {{\Delta\; P} = {\lambda \cdot \frac{L}{d} \cdot \frac{2G^{2}}{\rho}}} & (1) \end{matrix}$

In this formula, ΔP is the pressure loss [Pa], λ is a coefficient of friction loss, L is the flow passage length [m], d is the equivalent diameter of a flow passage [in], G is the mass velocity [kg/m²·s)], ρ is the working fluid density [kg/m³], and Re is a Reynolds number [−]. Further, the coefficient of friction loss λ is expressed by

λ=64/Re(Re<2300)

or

λ=0.3164·Re^(−0.25)(2300<Re),

depending on the range of values of the Reynolds number Re.

The equivalent diameter d of a flow passage is the diameter of a refrigerant flow passage in a case in which the refrigerant flow passage is circular in cross-section. In the case of a non-circular refrigerant flow passage, the equivalent diameter d is expressed by d=4A/I on the basis of the cross-sectional area of the refrigerant flow passage and the length of the edge of the cross-sectional shape of the refrigerant flow passage. At this time, A is the flow passage cross-sectional area [Pa], and I is the length [m] of a flow passage edge. The equivalent diameter d is the diameter of a cross-sectionally circular refrigerant flow passage equivalent to a cross-sectionally non-circular refrigerant flow passage.

As is seen from the aforementioned formula expressing the pressure loss ΔP, a narrower refrigerant flow passage or a longer refrigerant flow passage leads to a greater pressure loss.

Further, in a case in which the flow pattern of refrigerant is a two-phase gas-liquid state, a complex state in which liquid and gas are mixed together is brought about, so that there is an increase in pressure loss. Meanwhile, in the case of an embodiment such as the first expansion device 5 in which passage through a locally narrow flow part entails sharp decompression, the pressure loss ΔP is expressed basically in such a manner that a capacity coefficient Cv value peculiar to the shape of the first expansion device 5 is given. For example, in the case of a two-phase gas-liquid state at the inlet of the first expansion device 5, the pressure loss ΔP is expressed as follows:

[Math.  2]                                        $\begin{matrix} {{\Delta\; P} = {\frac{\rho}{\rho_{water}} \cdot \left( \frac{1 \cdot 17 \cdot Q}{Cv} \right)^{2}}} & (2) \end{matrix}$

In this formula, ΔP is the pressure loss [Pa], ρ is the working fluid density [kg/m³], ρ_(water) is the density of water [kg/m³] (fixed value), Q is the volumetric flow rate [m³/min], and Cv is the capacity coefficient [−]. Although other influences are technically taken into consideration for the pressure loss ΔP, the ratio between the circulatory volumes of refrigerant separately through parallel refrigerant flow passages formed by the refrigerant flow passage in which the first expansion device 5 is installed and the bypass 95 in which the second heat exchange unit 12 is installed is substantially determined by the aforementioned formula.

FIG. 3 is an explanatory diagram of the cross-sectional structure of the first heat exchange unit 11 and the second heat exchange unit 12 of the first heat exchanger 10 according to Embodiment 1. FIG. 3 shows a part of the cross-sectional structure of the first heat exchanger 10 in a cross-section passing through points A1, A2, A3, and A4 shown in FIG. 2. The cross-section passing through the points A1, A2, A3, and A4 is a cross-section parallel to an x-z plane. Further, FIG. 3 shows a state as seen from the direction of an arrow Y1 show in FIG. 2. That is, FIG. 3 shows a cross-section perpendicular to the tube axes of the heat transfer pipes 20. As shown in FIG. 3, the first heat exchanger 10 is formed by inserting the heat transfer pipes 20 into a plurality of notches 31 of fins 30 whose long sides extend in the z direction. The heat transfer pipes 20 have flat cross-sectional shapes whose major axes are oriented in an x direction and whose minor axes are oriented in the z direction. Air flows in the x direction into the first heat exchanger 10, passes between the fins 30 and the heat transfer pipes 20, and exchanges heat with refrigerant flowing through the heat transfer pipes 20.

FIG. 4 is an explanatory diagram of the structure of the first heat exchanger 10 according to Embodiment 1 as seen from the front. As shown in FIG. 4, an air current flowing into the first heat exchanger 10 during heating operation flows in a direction from the front toward the back of the drawing. The first heat exchanger 10 includes a plurality of heat transfer pipes 20 arranged in parallel with each other in the z direction with their tube axes oriented in the y direction. The plurality of heat transfer pipes 20 are, for example, flat pipes. The plurality of flat pipes are formed to have flat shapes having major axes and minor axes in cross-sections perpendicular to the tube axes. The plurality of flat pipes have their major axes oriented in the x direction.

FIG. 5 illustrates a cross-sectional view of a flat pipe as an example of a heat transfer pipe 20 for use in the first heat exchanger 10 of Embodiment 1. The flat pipe is made of a metal material having thermal conductivity. An example of the material of which the flat pipe is made is aluminum, an aluminum alloy, copper, or a copper alloy. The flat pipe is manufactured by extrusion by which internal flow passages 21 shown in FIG. 5 are shaped by forcing a heated material through holes of a die. Alternatively, the flat pipe may be manufactured by drawing by which a cross-section shown in FIG. 5 is formed by drawing the material out from holes of a die. A method for manufacturing the heat transfer pipe 20 is selectable as appropriate to the cross-sectional shape of the heat transfer pipe 20. The heat transfer pipe 20 is not limited to a flat pipe but may for example be a heat transfer pipe that is circular or elliptical in cross-section.

(Refrigeration Cycle Apparatus 1100 of Comparative Example)

FIG. 6 is a circuit diagram of a refrigerant circuit 101 of a refrigeration cycle apparatus 1100 as a comparative example of the refrigeration cycle apparatus 100 of Embodiment 1. FIG. 7 is a perspective view of a first heat exchanger 110 of the refrigeration cycle apparatus 1100 according to the comparative example. FIG. 7 partially schematically shows refrigerant pipes connected to the first heat exchanger 110. The refrigeration cycle apparatus 100 according to Embodiment 1 and the refrigeration cycle apparatus 1100 according to the comparative example differ in refrigerant circuit configuration downstream of the second heat exchanger 3 in the direction of refrigerant flow during heating operation.

As shown in FIG. 1, the refrigeration cycle apparatus 100 according to Embodiment 1 is configured such that one of the refrigerant pipes bifurcates downstream of the second heat exchanger 3, the first expansion device 5 and the second heat exchange unit 12 are disposed in parallel with each other, and flows of refrigerant having passed separately through the first expansion device 5 and the second heat exchange unit 12 merge and flow into the first heat exchange unit 11.

On the other hand, as shown in FIG. 6, the refrigeration cycle apparatus 1100 according to the comparative example is configured such that the first expansion device 5 and a second heat exchange unit 112 are connected in series downstream of the second heat exchanger 3, and refrigerant having passed through the first expansion device 5 and the second heat exchange unit 112 in sequence flows into a first heat exchange unit 111. As shown in FIG. 7, the numbers of refrigerant flow passages of the first heat exchange unit 111 and the second heat exchange unit 112 of the comparative example are set in a manner similar to those of the first heat exchanger 10 according to Embodiment 1.

FIG. 8 is a diagram showing the characteristics of the refrigeration cycle apparatus 1100 of the comparative example during heating operation. FIG. 8 is a P-h diagram showing changes in pressure and enthalpy of refrigerant during heating operation of the refrigeration cycle apparatus 1100. In the refrigeration cycle apparatus 1100 of the comparative example, high-pressure gas refrigerant (P₀₁) discharged from the compressor 2 flows into the second heat exchanger 3, which is an indoor heat exchanger, after passage through the four-way valve 7. It should be noted that a symbol in parentheses expressed by adding subscripts to “P” is a symbol shown in the P-h diagram of FIG. 8. The refrigerant has an enthalpy and a pressure represented by a point indicated by a symbol in parentheses.

The refrigerant having flowed into the second heat exchanger 3 is cooled (condensed) by exchanging heat with indoor air through the second heat exchanger 3. At this point in time, the temperature of the refrigerant is higher than the temperature of the indoor air. The refrigerant is cooled by the indoor air through the second heat exchanger 3, and turns into high-pressure liquid-phase refrigerant at the outlet of the second heat exchanger 3.

The high-pressure liquid refrigerant (P₁₁) having passed through the second heat exchanger 3 is decompressed by the first expansion device 5. The two-phase gas-liquid refrigerant (P₂₁) having passed through the first expansion device 5 flows into the second heat exchange unit 112, and is decompressed through a flow passage in a heat transfer pipe 20. In the diagram shown in FIG. 8, the refrigerant (P₂₁) having passed through the first expansion device 5 is in a two-phase gas-liquid state. In some cases, the refrigerant (P₂₁) having passed through the first expansion device 5 may be decompressed by the first expansion device 5 into medium-pressure single-phase liquid refrigerant.

The two-phase gas-liquid refrigerant (P₂₁) having passed through the first expansion device 5 flows into a heat transfer pipe 20 of the second heat exchange unit 112. As shown in FIG. 7, the second heat exchange unit 112 has a refrigerant flow passage formed by one heat transfer pipe 20. For this reason, the two-phase gas-liquid refrigerant passing through the second heat exchange unit 112 suffers from the pressure loss ΔP expressed by Formula (1) mentioned above. That is, the two-phase gas-liquid refrigerant passing through the second heat exchange unit 112 is decompressed.

In a case in which refrigerant is decompressed and undergoes a phase change from a single-phase liquid state to a two-phase gas-liquid state, the temperature of the refrigerant is determined by pressure. The temperature of the refrigerant is saturation temperature at a predetermined pressure. That is, as the two-phase gas-liquid refrigerant is decompressed, the two-phase gas-liquid refrigerant decreases in temperature accordingly. At this time, heat is exchanged in response to the temperature of a working fluid outside the heat transfer pipe 20. In a case in which the temperature of the refrigerant is higher than the temperature of the working fluid outside the pipe, the refrigerant is cooled (condensed) and the working fluid outside the pipe is heated. On the other hand, in a case in which the temperature of the refrigerant is lower than the temperature of the working fluid outside the pipe, the refrigerant is heated (evaporated) and the working fluid outside the pipe is cooled. In Embodiment 1, the working fluid outside the pipe is outside air.

The low-pressure two-phase refrigerant (P₃₁) having passed through the first expansion device 5 and the second heat exchange unit 112 is lower in temperature than the working fluid outside the pipe, and flows into the first heat exchange unit 111 and therefore becomes heated (evaporates). The refrigerant having flowed into the first heat exchange unit 111 evaporates in the first heat exchange unit 111, and the low-pressure gas refrigerant (P₄₁) passes through the four-way valve 7 and is suctioned into the compressor 2.

(Problems Presented by Refrigeration Cycle Apparatus 1100 of Comparative Example)

When, in the first heat exchanger 110 of the refrigeration cycle apparatus 1100 of the comparative example, the heat transfer pipes 20 are high in intratubular flow resistance, there are an increase in the pressure loss ΔP in the second heat exchange unit 112 and a decrease in pressure of the refrigerant at P₂₁. A case in which the heat transfer pipes 20 are high in flow resistance refers to a case in which refrigerant flow passages formed inside the heat transfer pipe 20 are thin, a case in which the refrigerant flow passages are long, or both of the cases. For example, when the internal flow passages 21 shown in FIG. 5 are thin, the pressure loss ΔP in the heat transfer pipe 20 increases. As indicated by Formula (1) mentioned above, a decrease in the equivalent diameter d of a flow passage and an increase in the flow passage length L lead to an increase in the pressure loss ΔP.

At this time, as shown in FIG. 8, an insufficient opening degree of the first expansion device 5 and a great pressure loss in the second heat exchange unit 112 may cause the pressure of the refrigerant (P₃₁) flowing into the first heat exchange unit 111 to be lower than it could possibly be in an ideal condition. That is, as shown in FIG. 8, the pressure of refrigerant flowing into the first heat exchange unit 111 of the first heat exchanger 110, which serves as evaporator, may become lower than a proper evaporator pressure P0. Such a state tends to be brought about in a case in which the number of refrigerant flow passages of the second heat exchange unit 112 is small and a refrigerant flow passage inside the heat transfer pipe 20 is thin and long.

As noted above, in the case of the first heat exchanger 110 of the refrigeration cycle apparatus 1100 of the comparative example, a problem exists in that the great difference in pressure between the suction port (P₄₁) and the discharge port (P₀₁) of the compressor 2 undesirably leads to an increase in amount of work of the compressor 2, and by extension to an increase in power consumption. This causes the refrigeration cycle apparatus 1100 to be low in efficiency, and impaired in power efficiency. Alternatively, in a case in which the temperature of refrigerant flowing through the inside of the first heat exchange unit 111 decreases as the pressure decreases and the first heat exchanger 110, which is used as outdoor heat exchanger, is operated at a low outside air temperature, the amount of frost formation increases, so that there may be a deterioration in heat exchanging performance.

Meanwhile, in a case in which a heat transfer pipe 20 used in an upper portion of the first heat exchanger 110 and a heat transfer pipe 20 used in a lower portion of the first heat exchanger 110 differ in type from each other and the cross-sectional area of a flow passage of the heat transfer pipe 20 used in the lower portion is large, a problem exists in that there is undesirably a deterioration in manufacturability of the first heat exchanger 110.

In FIG. 6, for operation with the pressure of the first heat exchanger 110 at a proper value P0, it is necessary to further increase the opening degree of the first expansion device 5. As the pressure loss ΔP in the second heat exchange unit 112 depends on the shape of the heat transfer pipe 20 of the second heat exchange unit 112, it is difficult to adjust to reduce the difference in pressure of refrigerant between the points P₂₁ and P₃₁ in FIG. 8 with the second heat exchange unit 112 alone. For a further increase in pressure of refrigerant at the point P₃₁ of FIG. 8, it is therefore necessary to increase the opening degree of the first expansion device 5 so that increased refrigerant flows through the first expansion device 5. That is, it is necessary to increase the opening degree of the first expansion device 5 to reduce the amount of decompression between the points P₁₁ and P₂₁ of FIG. 8. However, there is a limit to the range of adjustments to the opening degree of the first expansion device 5, such as electric expansion valve, a mechanical expansion valve, and a capillary tube, and in consideration of control of the refrigeration capacity of the refrigeration cycle apparatus 1100, a problem exists in that it is undesirably difficult to set a proper range of adjustments to the opening degree of the first expansion device 5. That is, an increase in flow passage resistance of the lower portion of the first heat exchanger 110 may make it impossible to make an adjustment for a necessary flow rate of refrigerant even at the maximum possible opening degree of the first expansion device 5, so that a problem exists in that there is undesirably a deterioration in controllability of the refrigeration cycle apparatus 1100.

(Workings of Refrigeration Cycle Apparatus 100 According to Embodiment 1)

FIG. 9 is a diagram showing the characteristics of the refrigeration cycle apparatus 100 according to Embodiment 1 during heating operation. FIG. 10 is an enlarged view of an A part of FIG. 9. FIG. 9 is a P-h diagram showing changes in pressure and enthalpy of refrigerant during heating operation of the refrigeration cycle apparatus 100. In the refrigeration cycle apparatus 100, the high-pressure gas refrigerant (P₀₁) discharged from the compressor 2 passes through the four-way valve 7 and flows into the second heat exchanger 3, which is an indoor heat exchanger. The refrigerant is cooled (condensed) by exchanging heat with indoor air. At this point in time, the temperature of refrigerant is higher than that of the indoor air. The refrigerant is cooled by the indoor air through the second heat exchanger 3, and turns into high-pressure liquid-phase refrigerant at the outlet of the second heat exchanger 3.

The high-pressure liquid refrigerant (P₁₁) having passed through the second heat exchanger 3 bifurcates into two flows of refrigerant that are distributed separately to the second heat exchange unit 12 and the first expansion device 5 and expanded, that is, decompressed. As is the case with the refrigerant having flowed into the second heat exchange unit 112 in the comparative example, the refrigerant having flowed into the second heat exchange unit 12 is decompressed by the refrigerant flow passage in the heat transfer pipe 20. In a case in which the refrigerant is decompressed in the heat transfer pipe 20 and undergoes a phase change from a single-phase liquid state to a two-phase gas-liquid state, the temperature of the refrigerant is determined by pressure. That is, as the refrigerant is decompressed, the refrigerant also decreases in temperature. At this time, the refrigerant flowing through the heat transfer pipe 20 and the outside air exchange heat with each other in response to the temperature of the working fluid outside the heat transfer pipe 20, that is, the outside air. In a case in which the temperature of the refrigerant is higher than the temperature of the working fluid outside the pipe, the refrigerant is cooled (condensed) and the working fluid outside the pipe is heated. On the other hand, in a case in which the temperature of the refrigerant is lower than the temperature of the working fluid outside the pipe, the refrigerant is heated (evaporated) and the working fluid outside the pipe is cooled. As a result, the refrigerant flowing through the second heat exchange unit 12 turns into low-pressure two-phase gas-liquid refrigerant (P₂₂).

The refrigerant having flowed into the first expansion device 5 is expanded (decompressed) and turns into low-pressure two-phase gas-liquid refrigerant (P₂₁). At this time, as the first expansion device 5 effects adiabatic expansion, which does not involve heat exchange of refrigerant, the value of enthalpy of the two-phase gas-liquid refrigerant (P₂₁) is the same as it was before the expansion (P₁₁).

The ratio between the circulatory volumes of separate flows of refrigerant through the second heat exchange unit 12 and the first expansion device 5 is uniformly determined by the difference between the magnitude of flow resistance in the heat transfer pipes 20 of the second heat exchange unit 12 and the magnitude of flow resistance by throttling of the first expansion device 5.

The pressure loss ΔP of a heat transfer pipe 20 is calculated by Formula (1) mentioned above. In Formula (1), the coefficient of friction loss λ the flow passage length L, and the equivalent diameter d of a flow passage are determined by the shape of a heat transfer pipe 20 and the number of heat transfer pipes 20 that the second heat exchange unit 12 has. Meanwhile, in Formula (1), the mass velocity G is determined by the amount of refrigerant that flows into the second heat exchange unit 12, and the working fluid density p varies depending on whether the refrigerant is single-phase refrigerant or two-phase gas-liquid refrigerant. Meanwhile, the pressure loss ΔP of the first expansion device 5 is determined by Formula (2). In a case in which the opening degree is small (i.e. a case in which Cv is small), the flow rate is low and the pressure loss ΔP is great. In a case in which the opening degree is large (i.e. a case in which Cv is large), the flow rate is high and the pressure loss ΔP is small.

Therefore, in the refrigerant circuit 1, the decompression of refrigerant in a section in which the second heat exchange unit 12 and the first expansion device 5 are connected in parallel with each other, that is, the decompression of refrigerant in a section from P₁₁ to P₃₁, is controllable by the opening degree of the first expansion device 5.

The low-pressure two-phase gas-liquid refrigerant (P₂₂) having passed through the second heat exchange unit 12 and the low-pressure two-phase gas-liquid refrigerant (P₂₁) having passed through the first expansion device 5 merge into low-pressure two-phase refrigerant (P₃₁) corresponding to the ratio between the circulatory volumes and the respective enthalpies of the refrigerant, and the low-pressure two-phase refrigerant (P₃₁) flows into the first heat exchange unit 11 and is heated (evaporated). The low-pressure gas refrigerant (P₄₁) having evaporated in the first heat exchange unit 11 passes through the four-way valve 7 and is suctioned into the compressor 2.

(Effects of Embodiment 1)

Even in a case in which the intratubular flow resistance of the heat transfer pipes 20 of the second heat exchange unit 12 is high, the refrigeration cycle apparatus 100 according to Embodiment 1 includes the bypass 95 in parallel with the refrigerant flow passage in which the first expansion device 5 is installed. For this reason, the flow resistance of the refrigerant flow passage in a portion of the refrigerant circuit 1 in which the second heat exchange unit 12 and the first expansion device 5 are parallel with each other is lower than that in a case in which the second heat exchange unit 12 or the first expansion device 5 are each independently installed in series. This eliminates the need to increase the opening degree of the first expansion device 5, and the opening degree of the first expansion device 5 is no longer insufficient. This also allows high-pressure liquid refrigerant higher in temperature than the indoor air to flow into the second heat exchange unit 12 including the lowermost part of the first heat exchanger 10. This makes it possible to reduce freezing of drainage water accumulated in a lower part of the first heat exchanger 10.

A refrigeration cycle apparatus 100 according to Embodiment 1 includes a refrigerant circuit 1 connecting a compressor 2, a first heat exchanger 10, and a first expansion device 5 by refrigerant pipes. The first heat exchanger 10 includes a first heat exchange unit 11 and a second heat exchange unit 12 connected to the first heat exchange unit 11 in series in the refrigerant circuit 1. The first expansion device 5 is connected in parallel with the second heat exchange unit 12 in the refrigerant circuit 1, and the second heat exchange unit 12 is placed at a position lower than a position of the first heat exchange unit 11.

In a case in which the first heat exchanger 10 serves as evaporator, refrigerant having flowed out from the second heat exchanger 3 is distributed to the first expansion device 5 and the second heat exchange unit 12 first. For this reason, the refrigerant flows through the second heat exchange unit 12 in a range of saturation temperatures in conformance with the difference in pressure between a portion upstream and a portion downstream of the first expansion device 5 of the refrigerant circuit 101 of the comparative example, which has been used. That is, as the second heat exchange unit 12 according to Embodiment 1 is higher in refrigerant temperature than the inlet of the first heat exchanger 110, which is used as evaporator, of the refrigerant circuit 101 of the comparative example, freezing of accumulated water in the lowermost part of the first heat exchanger 10, which is used as evaporator, is reduceable.

Further, the second heat exchange unit 12 is installed in the bypass 95, which bypasses the first expansion device 5. Adding the second heat exchange unit 12 in parallel with the first expansion device 5 makes it possible to make the maximum opening degree of the first expansion device 5 smaller than that in the refrigerant circuit 101, such as the comparative example, which connects the first expansion device 5 and the second heat exchange unit 12 in series. Therefore, in a case in which the pressure loss ΔP of refrigerant passing through the second heat exchange unit 12 is great, the first expansion device 5 hardly suffers from an insufficient opening degree, and the range is widen within which the pressure of refrigerant in the evaporator is controllable.

In particular, in a case in which flat pipes are employed as the heat transfer pipes 20 of the first heat exchanger 10, the refrigerant flow passages may be so thin that a great pressure loss is incurred when refrigerant is passed through the refrigerant flow passages. For reductions in the amounts of refrigerant in the first heat exchanger 10 and the refrigerant circuit 1, it is desirable that the heat transfer pipes 20 have thinly-formed refrigerant flow passages, and for example, it is desirable that the heat transfer pipes 20 be flat pipes with a thickness less than or equal to 1 mm or, more desirably, less than or equal to 0.8 mm in a direction of the minor axes. At this time, in the case of a need to increase the refrigerant pressure of the first heat exchanger 10, which serves as evaporator, that is, in the case of a need for operation with the evaporator low in heat exchanging performance, the pressure loss ΔP in the second heat exchange unit 112, which is located in the lower part of the first heat exchanger 10, is high in the refrigerant circuit 101 of the comparative example. For this reason, a problem exists in that unless the opening degree of the first expansion device 5 is great, the pressure in the first heat exchange unit 111 will be lower than the proper evaporator pressure P0. Meanwhile, in the refrigerant circuit 1 of the refrigeration cycle apparatus 100 according to Embodiment 1, the second heat exchange unit 12, which is great in pressure loss, and the first expansion device 5 are disposed in parallel with each other, so that the pressure in the evaporator is properly controllable without widening the range of opening degree of the first expansion device 5.

Further, as the first heat exchange unit 11 and the second heat exchange unit 12 of the first heat exchanger 10 are formed in an integrated manner, there is such an advantage that the first heat exchanger 10 is manufactured with improved ease of assembly.

In the refrigeration cycle apparatus 100 according to Embodiment 1, the first heat exchange unit 11 has a larger number of refrigerant flow passages than does the second heat exchange unit 12. As the first heat exchanger 10 is formed by two elements, namely the first heat exchange unit 11 and the second heat exchange unit 12, and the first heat exchange unit 11 and the second heat exchange unit 12 are connected in series, the pressure loss ΔP of the first heat exchanger 10 is increasable. In particular, in a case in which the first heat exchanger 10 is used as evaporator, the pressure loss ΔP in the second heat exchange unit 12 is increasable by making the number of refrigerant path bifurcations of the second heat exchange unit 12 upstream of the first heat exchange unit 11 in the flow of refrigerant smaller than the number of refrigerant path bifurcations of the first heat exchange unit 11. This makes it possible to, while reducing freezing of accumulated water in the lowermost part of the first heat exchanger 10 and without providing an additional expansion device downstream of the second heat exchange unit 12, reduce the pressure of refrigerant flowing into the first heat exchange unit 11.

In the refrigeration cycle apparatus 100 according to Embodiment 1, a heat transfer pipe 20 that the first heat exchange unit 11 includes is disposed parallel to a heat transfer pipe 20 that the second heat exchange unit 12 includes. Thus, in the first heat exchanger 10, high-temperature refrigerant flows through a lower heat transfer pipe 20 on which droplets of water falling from an upper heat transfer pipe 20 tend to accumulate. This makes it possible to reduce freezing of accumulated water accumulating on an upper surface of a heat transfer pipe 20.

In the refrigeration cycle apparatus 100 according to Embodiment 1, each of the heat transfer pipes 20 is a flat pipe. As the heat transfer pipes 20 that the second heat exchange unit 12, which is located in the lower part of the first heat exchanger 10, has are flat pipes, the pressure of refrigerant passing through the second heat exchange unit 12 is easily reduced. Accordingly, as high-temperature refrigerant flows through the lower part of the first heat exchanger 10 while the pressure of refrigerant is reduced through the second heat exchange unit 12, which is disposed in the bypass 95, which does not pass through the first expansion device 5, freezing in the lower part of the first heat exchanger 10 is reduceable. Further, as the heat transfer pipes 20 are flat pipes, the volume of refrigerant in the first heat exchanger 10 is reduceable while heat exchanging performance is maintained or improved, so that the amount of refrigerant that flows through the refrigerant circuit 1 is reduceable.

Embodiment 2

A refrigeration cycle apparatus 100 according to Embodiment 2 is one obtained by further adding an expansion device to the refrigerant circuit 1 of the refrigeration cycle apparatus 100 according to Embodiment 1. The refrigeration cycle apparatus 200 according to Embodiment 2 is described with a focus on changes made to Embodiment 1. Components of the refrigeration cycle apparatus 200 according to Embodiment 2 that have the same functions as those of Embodiment 1 are shown in each drawing with reference to the same reference signs as those of the drawings used to describe Embodiment 1.

FIG. 11 is a circuit diagram of a refrigerant circuit 201 of the refrigeration cycle apparatus 200 according to Embodiment 2. FIG. 12 is a perspective view of a first heat exchanger 210 of the refrigeration cycle apparatus 200 according to Embodiment 2. The refrigerant circuit 201 of the refrigeration cycle apparatus 200 according to Embodiment 2 is one obtained by adding a second expansion device 51 between the second heat exchange unit 12 and the first heat exchange unit 11 of the first heat exchanger 10 of the refrigeration cycle apparatus 100 according to Embodiment 1. The second expansion device 51 is disposed closer to the second heat exchange unit 12 than a confluence 91 at which the flow passages, separated at the bifurcation 90, in which the first expansion device 5 and the second heat exchange unit 12 are disposed merge. In other words, a bypass 295 connecting the second heat exchange unit 12 and the second expansion device 51 in series is connected in parallel with the first expansion device 5,

FIG. 13 is a diagram showing the characteristics of the refrigeration cycle apparatus 200 according to Embodiment 2 during heating operation. FIG. 13 is a P-h diagram showing changes in pressure and enthalpy around a low-temperature and low-pressure region of the refrigeration cycle apparatus 200. In the refrigeration cycle apparatus 100 of Embodiment 1, the pressure loss ΔP in the second heat exchange unit 12 is small, depending on the specifications of the second heat exchange unit 12, so that the pressure of refrigerant having just left the second heat exchange unit 12 may be high. That is, as indicated by the point P₂₃ in FIG. 13, the refrigerant having left the second heat exchange unit 12 may be higher in temperature than outdoor air. Therefore, the refrigerant having left the second heat exchange unit 12 is further decompressed by the second expansion device 51 to a pressure lower than a pressure corresponding to outdoor air temperature. With this configuration, the refrigeration cycle apparatus 200 is configured to properly set or control the pressure of the first heat exchanger 210, which is used as evaporator. Further, at this time, the temperature of the refrigerant having flowed out from the second heat exchange unit 12 is higher than the outside air temperature. Therefore, even in a low-outside-air-temperature environment in which the outdoor air temperature is close to the freezing point of water, high-temperature refrigerant flows through the second heat exchange unit 12. This makes it possible to reduce frost formation and freezing.

FIG. 14 is a diagram showing the characteristics of the refrigeration cycle apparatus 200 according to Embodiment 2 during heating operation. FIG. 14 is a P-h diagram showing changes in pressure and enthalpy around a low-temperature and low-pressure region of the refrigeration cycle apparatus 200. FIG. 14 is a diagram of a case in which the pressure loss ΔP in the second heat exchange unit 12 is greater than that in the case of FIG. 13. At this time, the refrigerant having flowed out from the second heat exchange unit 12 is lower in temperature than the outdoor air. For this reason, a portion of the refrigerant about to flow out from the second heat exchange unit 12 is lower in temperature than the outdoor air, so that frost may form or accumulated water may freeze in an area around the outlet of the heat transfer pipes 20 of the second heat exchange unit 12. However, as the refrigeration cycle apparatus 200 according to Embodiment 2 includes the second expansion device 51, the opening degree of the second expansion device 51 is settable or controllable depending on the outdoor air temperature such that the temperature at the point P₂₃ does not fall below the freezing point of water. This makes it possible to reduce the occurrence of frost formation and freezing in only a portion of the area around the outlet of the second heat exchange unit 12.

The first expansion device 5 and the second expansion device 51 are not limited solely to expansion devices with variable opening degrees, but may be expansion devices with fixed opening degrees. Further, at least either the first expansion device 5 or the second expansion device 51 may be an expansion device with a variable opening degree.

In the refrigeration cycle apparatus 200 according to Embodiment 2, the second expansion device 51 is connected in parallel with the first expansion device 5 and connected to the second heat exchange unit 12 in series in the refrigerant circuit 201.

The refrigerant having passed through the second heat exchange unit 12 is decompressed by the second expansion device 51 and therefore rises in refrigerant pressure and refrigerant temperature at a portion upstream of the second expansion device 51, that is, at a portion close to the outlet of the second heat exchange unit 12. Therefore, the refrigerant temperature is kept high throughout the second heat exchange unit 12. For this reason, the first heat exchanger 210 more easily reduces freezing of accumulated water in the lower part of the first heat exchanger 210 than does the first heat exchanger 10 according to Embodiment 1.

Further, for example, in an operational state, such as the case of low-load capacity operation of the refrigeration cycle apparatus 200, in which the circulatory volume of refrigerant needs to be reduced, it is necessary to perform operation with the opening degree of the first expansion device 5 reduced to zero. However, in a case in which the flow passage resistance of the second heat exchange unit 12 is low, the amount of refrigerant that flows to the second heat exchange unit 12 increases. Alternatively, an insufficient resolution with which the opening degree of the first expansion device 5 is set may make it impossible to properly set the pressure of refrigerant flowing into the first heat exchange unit 11, with the result that the refrigeration cycle apparatus 200 can no longer be set or controlled to target low-load capacity. The case in which the flow passage resistance of the second heat exchange unit 12 is low is for example a case in which the pressure loss ΔP in the heat transfer pipes 20 of the second heat exchange unit 12 is small.

By including the bypass 295, which connects the second heat exchange unit 12 and the second expansion device 51 in series, the refrigeration cycle apparatus 200 according to Embodiment 2 makes it possible to add flow passage resistance to a part of the bypass 295 beside the second heat exchange unit 12. That is, the second expansion device 51, which is installed in the bypass 295, as well as the first expansion device 5 is used to control the pressure of the refrigerant flowing into the first heat exchange unit 11. For this reason, the refrigeration cycle apparatus 200 is configured to better improve the pressure control capability of the first heat exchanger 10, which is configured to serve as evaporator during operation in a low-load capacity state, than the refrigeration cycle apparatus 100 according to Embodiment 1.

Embodiment 3

A refrigeration cycle apparatus 300 according to Embodiment 3 is one obtained by further adding an expansion device to the refrigerant circuit 1 of the refrigeration cycle apparatus 100 according to Embodiment 1. The refrigeration cycle apparatus 300 according to Embodiment 3 is described with a focus on changes made to Embodiment 1. Components of the refrigeration cycle apparatus 300 according to Embodiment 3 that have the same functions as those of Embodiment 1 are shown in each drawing with reference to the same reference signs as those of the drawings used to describe Embodiment 1.

FIG. 15 is a circuit diagram of a refrigerant circuit 301 of the refrigeration cycle apparatus 300 according to Embodiment 3. FIG. 16 is a perspective view of a first heat exchanger 310 of the refrigeration cycle apparatus 300 according to Embodiment 3. The refrigerant circuit 301 of the refrigeration cycle apparatus 300 according to Embodiment 3 is one obtained by adding a second expansion device 52 between the second heat exchange unit 12 and the first heat exchange unit 11 of the first heat exchanger 10 of the refrigeration cycle apparatus 100 according to Embodiment 1. The second expansion device 52 is disposed closer to the first heat exchange unit 11 than a confluence 91 at which the flow passages, separated at the bifurcation 90, in which the first expansion device 5 and the second heat exchange unit 12 are disposed merge. In other words, the second expansion device 52 is connected to the first expansion device 5 in series and connected to the second heat exchange unit 12 in series.

FIG. 17 is a diagram showing the characteristics of the refrigeration cycle apparatus 300 according to Embodiment 3 during heating operation. FIG. 17 is a P-h diagram showing changes in pressure and enthalpy around a low-temperature and low-pressure region of the refrigeration cycle apparatus 300. In the refrigeration cycle apparatus 100 of Embodiment 1, the pressure loss ΔP in the second heat exchange unit 12 is small, depending on the specifications of the second heat exchange unit 12, so that the pressure of refrigerant having just left the second heat exchange unit 12 may be high. That is, as indicated by the point P₂₂ in FIG. 17, the refrigerant having left the second heat exchange unit 12 may be higher in temperature than outdoor air.

Further, depending on the capacity or opening-degree resolution of the first expansion device 5, the pressure of refrigerant having flowed out from the first expansion device 5, that is, the pressure of refrigerant at the point P₂₁, may not be sufficiently reduced. Therefore, the interflow of refrigerant having left the second heat exchange unit 12 and refrigerant having left the first expansion device 5 is further decompressed by the second expansion device 52 to a pressure lower than a pressure corresponding to outdoor air temperature. With this configuration, the refrigeration cycle apparatus 300 according to Embodiment 3 is configured to properly set or control the pressure of the first heat exchanger 310, which is used as evaporator.

In the characteristics of the refrigeration cycle apparatus 300 during heating operation as shown in FIG. 17, the pressure and temperature of refrigerant at the outlet of the second heat exchange unit 12 are kept high, so that the refrigerant temperature is kept high throughout the second heat exchange unit 12. This brings about such an advantage that freezing of accumulated water in the lowermost part of the first heat exchanger 10 is easily reduced as in the case of the first heat exchanger 210 according to Embodiment 2.

FIG. 18 is a diagram showing the characteristics of the refrigeration cycle apparatus 300 according to Embodiment 3 during heating operation. FIG. 18 is a P-h diagram showing changes in pressure and enthalpy around a low-temperature and low-pressure region of the refrigeration cycle apparatus 200. FIG. 18 is a diagram of a case in which the pressure loss ΔP in the second heat exchange unit 12 is greater than that in the case of FIG. 17. At this time, the refrigerant having flowed out from the second heat exchange unit 12 is lower in temperature than the outdoor air. For this reason, a portion of the refrigerant about to flow out from the second heat exchange unit 12 is lower in temperature than the outdoor air, so that frost may form or accumulated water may freeze in an area around the outlet of the heat transfer pipes 20 of the second heat exchange unit 12. However, as the refrigeration cycle apparatus 300 according to Embodiment 3 includes the second expansion device 52, the opening degree of the second expansion device 52 is settable or controllable depending on the outdoor air temperature such that the temperature at the point P₃₂ does not fall below the freezing point of water. This makes it possible to reduce the occurrence of frost formation and freezing in only a portion of the area around the outlet of the second heat exchange unit 12.

Also in Embodiment 3, the first expansion device 5 and the second expansion device 52 are not limited solely to expansion devices with variable opening degrees, but may be expansion devices with fixed opening degrees. Further, at least either the first expansion device 5 or the second expansion device 52 may be an expansion device with a variable opening degree.

Embodiment 4

A refrigeration cycle apparatus 400 according to Embodiment 4 is one obtained by changing the structure of the first heat exchanger 10 of the refrigeration cycle apparatus 100 according to Embodiment 1. The refrigeration cycle apparatus 400 according to Embodiment 4 is described with a focus on changes made to Embodiment 1. Components of the refrigeration cycle apparatus 400 according to Embodiment 4 that have the same functions as those of Embodiment 1 are shown in each drawing with reference to the same reference signs as those of the drawings used to describe Embodiment 1.

FIG. 19 is a circuit diagram of a refrigerant circuit 401 of the refrigeration cycle apparatus 400 according to Embodiment 4. FIG. 20 is a perspective view of a first heat exchanger 410 of the refrigeration cycle apparatus 400 according to Embodiment 4. The refrigerant circuit 401 of the refrigeration cycle apparatus 400 according to Embodiment 4 is one obtained by dividing the first heat exchange unit 11 of the first heat exchanger 10 of the refrigeration cycle apparatus 100 according to Embodiment 1. In the first heat exchange unit 11 according to Embodiment 1, the plurality of heat transfer pipes 20 are all in parallel with each other, and refrigerant flows into all of the plurality of heat transfer pipes 20 at the same time. On the other hand, in the first heat exchange unit 11 according to Embodiment 4, a plurality of heat transfer pipes 20 located in a lower part 16 of the first heat exchange unit 11 and a plurality of heat transfer pipes 20 located in an upper part 15 of the first heat exchange unit 11 are connected in series.

FIG. 21 is a diagram showing the characteristics of the refrigeration cycle apparatus 400 according to Embodiment 4 during heating operation. FIG. 21 is a P-h diagram showing changes in pressure and enthalpy around a low-temperature and low-pressure region of the refrigeration cycle apparatus 400. In the refrigeration cycle apparatus 100 of Embodiment 1, the pressure loss ΔP in the second heat exchange unit 12 is small, depending on the specifications of the second heat exchange unit 12, so that the pressure of refrigerant having just left the second heat exchange unit 12 may be high. That is, as indicated by the point P₂₂ in FIG. 21, the refrigerant having left the second heat exchange unit 12 may be higher in temperature than outdoor air.

Further, depending on the capacity or opening-degree resolution of the first expansion device 5, the pressure of refrigerant at the point P₂₁ may not be sufficiently reduced. Therefore, the interflow of refrigerant having left the second heat exchange unit 12 and refrigerant having left the first expansion device 5 needs to be further decompressed by the lower part 16 of the first heat exchange unit 11 to a pressure lower than a pressure corresponding to outdoor air temperature. With this configuration, the refrigeration cycle apparatus 400 is configured to properly set or control the pressure of the first heat exchanger 410, which is used as evaporator.

Such a configuration makes it possible to supply high-temperature refrigerant to the lower part 16 of the first heat exchange unit 11 as well as the second heat exchange unit 12 in such a case in which the temperature of outside air around the first heat exchanger 410, which is used as evaporator, is close to the freezing point of water or lower than or equal to the freezing point.

While the present disclosure has been described above with reference to embodiments, the present disclosure is not limited solely to the configurations of the aforementioned embodiments. For example, while the first heat exchangers 10, 210, and 310 according to Embodiments 1 to 3 have been described as being structured to be divided into two separate parts, namely the first heat exchange unit 11 and the second heat exchange unit 12, the heat exchange units may each be divided as appropriate. For example, the first heat exchange unit 11 and the second heat exchange unit 12 may each be divided into the same number of separate parts, and the separated parts may be connected in series. Furthermore, the present disclosure may be made by a combination of one embodiment and another. In other words, a range of various changes, applications, and utilizations made by a person skilled in the art as needed is encompassed in the scope (technical scope) of the present disclosure.

REFERENCE SIGNS LIST

1: refrigerant circuit, 2: compressor, 3: second heat exchanger, 4: air-sending device, 5: first expansion device, 6: air-sending device, 7: four-way valve, 10: first heat exchanger, 11: first heat exchange unit, 11 a: first windward heat exchange unit, 11 b: first leeward heat exchange unit, 12: second heat exchange unit, 12 a: second windward heat exchange unit, 12 b: second leeward heat exchange unit, 13 a: (windward) collecting pipe, 13 b: (leeward) collecting pipe, 14: header, 14 a: upper header, 14 b: lower header, 15: upper part, 16: lower part, 20: heat transfer pipe, 21: internal flow passage, 30: fin, 31: notch, 51: second expansion device, 52: second expansion device, 30: refrigerant pipe, 90: bifurcation, 91: confluence, 95: bypass, 100: refrigeration cycle apparatus, 101: refrigerant circuit, 110: first heat exchanger, 111: first heat exchange unit, 112: second heat exchange unit, 200: refrigeration cycle apparatus, 201: refrigerant circuit, 210: first heat exchanger, 295: bypass, 300: refrigeration cycle apparatus, 301: refrigerant circuit, 310: first heat exchanger, 400: refrigeration cycle apparatus, 401: refrigerant circuit, 410: first heat exchanger, 1100: refrigeration cycle apparatus, 0: mass velocity, P0: evaporator pressure, Re: Reynolds number, Y1: arrow, d: equivalent diameter, ΔP: pressure loss, λ: coefficient of friction loss, ρ: working fluid density 

1. A refrigeration cycle apparatus, comprising a refrigerant circuit connecting, by refrigerant pipes, a compressor, a first expansion device, and a first heat exchanger configured to serve as evaporator during heating operation, the first heat exchanger being provided with a first heat exchange unit, a second heat exchange unit connected to the first heat exchange unit in series in the refrigerant circuit; and a second expansion device connected between the first heat exchange unit and the second heat exchange unit, the first expansion device being connected in parallel with the second heat exchange unit in the refrigerant circuit, the second heat exchange unit being placed at a position lower than a position of the first heat exchange unit, the second expansion device being connected in parallel with the first expansion device and connected to the second heat exchange unit in series in the refrigerant circuit. 2-4. (canceled)
 5. The refrigeration cycle apparatus of claim 1, wherein the first heat exchange unit has a larger number of refrigerant flow passages than does the second heat exchange unit.
 6. The refrigeration cycle apparatus of claim 1, wherein the first heat exchanger includes a plurality of heat transfer pipes arranged in parallel with each other in a vertical direction, and each of the plurality of heat transfer pipes is a flat pipe.
 7. The refrigeration cycle apparatus of claim 1, wherein the refrigerant circuit includes a second heat exchanger configured to serve at least partially as condenser during heating operation, and a bifurcation at which one of the refrigerant pipes bifurcates into one to the second heat exchanger and an other one to the first expansion device. 